Organic peroxides are commonly used to crosslink thermoplastic polymers and elastomers, and their mixtures, when the final products are required to meet high mechanical and physical requirements, such as improved thermal aging and lower residual (permanent) deformation under pressure compared with uncrosslinked thermoplastics and/or sulfur cured elastomers. Since both thermoplastics and elastomers are solid materials at room temperatures, in order to add the free radical crosslinking agent and any other desired ingredients such as dyes, pigments, fillers, antioxidants, UV and heat stabilizers and the like into the polymer, the polymers must be mechanically mixed with the free radical crosslinking agent and any of the other desired ingredients at temperatures sufficiently elevated to allow the polymers to flow in the mixing equipment.
The crosslinking period and the time from the addition of the free radical precursor up to incipient cross linkage (scorch time) are dependent on the thermal decomposition rate (conveniently expressed as the half-life period) of the free radical initiators employed as crosslinking agents. The longer the processing time before onset of scorch that can be provided to a manufacturer without sacrifice of final crosslink speed or density, the more beneficial it will be because with conventional methods of mixing or compounding, such as milling, Banbury, or extrusion, scorch begins when the time and temperature relationship results in the start of appreciable decomposition of the free radical initiator. If this occurs too soon, gel particles in the mass of polymer to be compounded may be formed thereby producing non-homogeneity in the final product. Excessive scorch reduces the plastic properties of the material so that it can no longer be processed, thus, resulting in loss of the entire batch.
There have been several attempts to extend scorch time. U.S. Pat. No. 5,245,084 discloses the use of organic peroxides suitable for crosslinking thermoplastics and elastomers in combination with a specific group of hydroquinones and a crosslinkage promoter selected from crosslinkage promoters normally used in these applications. U.S. Pat. No. 6,197,231 teaches the use of a combination of free radical initiators (either organic peroxides or a specific class of azo initiators) in combination with hydroquinones, crosslinkage promoters and known sulfur releasing sulfur accelerators for extending scorch time without adverse effects on cure time or cure density for thermoplastics, elastomers and their mixtures.
There are several commercial grades of extended organic peroxide formulations on fillers. However, due to the poor solubility of solid additives in organic peroxides, it has not been possible to create a liquid peroxide formulation or homogeneous meltable solid that provides sufficient scorch protection and contains either no filler or lower amounts of filler. It would be desirable to have organic peroxide formulations with longer scorch times and lower loadings of additives.